The present invention relates to fast channel switching systems, and more particularly, to an input current cancellation scheme in fast channel switching systems.
As electronic systems continue to grow in complexity, many devices, such as communication and medical equipment, simultaneously receive and process data signals originating from multiple data channels. For example, advanced communication protocols, such as Long Term Evolution (LTE), make use of multiple antennas to implement multiple input multiple output (MIMO) techniques on both mobile devices and base stations to achieve increased data rates. In such MIMO systems, multiple signals can be received and processed in parallel. In another example, medical devices, such as ultrasound, process data from multiple sensors to generate a single output, such as an image.
As can be expected, the analog to digital converter (ADC) a fundamental component of these electronic devices. Referring to the examples above, the ADC typically receives analog signals from a signal source (e.g., an antenna or a sensor), which are then converted by the ADC to a digital code representing the data. Accordingly, in multichannel receivers, each parallel analog signal is typically processed by one or more ADCs to produce an output digital signal. To convert multichannel analog signals to digital form, several circuit architectures have been developed.
In a related art approach, a separate ADC is included for each input channel. In addition to the high component cost associated with including an ADC on each channel, the power consumption of this arrangement is also significantly increased.
In another related art approach, a front-end multiplexer is employed such that each input channel is coupled to the front-end multiplexer, and the output of the multiplexer is relayed to the ADC and the rest of the circuitry in the chain. A drawback of this scheme is that it draws considerable input current from the input driver which becomes a concern as the speed of the switching increases. In addition, capacitance on the output of the multiplexer can cause input currents when switching channels.
To increase the switching speed of ADCs, input buffers can be used to charge an ADC input capacitor in order to reduce residual input current. Residual input current has undesirable effects including undesired gain, offset, and nonlinearity issues. The input buffer may have an offset that can be cancelled out by using a chopping mechanism. These techniques, however, reduce the accuracy of the ADC's output.
In yet another related art approach, the multiplexer output and the ADC input are decoupled and an external buffer is disposed at the front-end of the ADC. Here, the capacitance on the output side of the multiplexer can be particularly high (i.e., compared to internal loads) which results in high input currents at fast channel-switching rates.
As related art techniques either have high power consumption or low switching rates, the inventors perceive a need for a multichannel system that overcomes the above-identified drawbacks of existing systems.